Final answer:
The ΔHrxn represents an enthalpy of formation when it refers to the enthalpy change involved in the formation of one mole of a compound from its elements in their most stable forms.
Step-by-step explanation:
The question asks for which reaction does ΔHrxn represent an enthalpy of formation. The standard enthalpy of formation is defined for the formation of a compound from its elements in their standard states.
Using Hess's Law, the standard enthalpy change of a reaction can be found by summing the standard enthalpies of formation (ΔH°f) of the products and subtracting the sum of the standard enthalpies of formation of the reactants.
Typically, the reaction for an enthalpy of formation involves forming one mole of a compound from its elements in their most stable forms.
For instance, the standard enthalpy of formation for ferric chloride (FeCl3(s)) would be the enthalpy change for the reaction using elemental iron (Fe) and chlorine gas (Cl2) to form FeCl3.